A capacitive MEMS pressure sensor requires two electrodes that move relative to each other under an applied pressure. This configuration is most often accomplished by having a fixed electrode (hereafter called the lower electrode) formed in a substrate while a moveable electrode (hereafter called the upper electrode) is provided in a deformable membrane which is exposed to the pressure that is to be sensed. One or more of the electrodes are typically formed by deposition of a conductive film, electrical isolation of a conductive layer, or by adding an oxide layer between two conductive materials.
FIG. 1 depicts a side cross-sectional view of a MEMS pressure sensor 10 such as described in U.S. patent application Ser. No. 13/232,005 which was filed on Sep. 14, 2011, the entire contents of which are herein incorporated by reference. The pressure sensor 10 includes a pair of electrodes that are configured to move relative to one another. The pressure sensor 10 includes a handle layer 12, a buried oxide layer 14, and a device layer 16. An oxide layer 18 separates the device layer 16 from a cap layer 20. A passive layer 22 is located above the cap layer 20.
Within the device layer 16, an in-plane electrode 24 is defined by two etch portions 26 and 28. The in-plane electrode 24 is isolated from the cap layer 20 by an etched portion 30 of the oxide layer 18. The etched portions 26, 28, and 30 are etched through vent holes 32 which are closed by the cap layer 20.
An out-of plane electrode 34 is located above the in-plane electrode 24 and electrically isolated from the in-plane electrode 24 by the etched portion 30. The out-of-plane electrode 34 is isolated from the rest of the cap layer 20 by two spacers 36 and 38. The spacers 36 and 38 include a lower nitride portion 40 which extends upwardly from the etched portion 30, and an upper oxide portion 42 which extends from the nitride portion 40 to the upper surface of the cap layer 20.
Spacers 44 and 46, which are formed like the spacers 36 and 38, electrically isolate a connector 48 in the cap layer 20 from the rest of the cap layer 20. The connector 48 is in electrical communication with a connector 50 in the device layer 16. The connector 50 is in electrical communication with the in-plane electrode 24 and isolated from the remainder of the device layer 16 by isolation posts 52 and 54. The isolation posts 52 and 54 extend from the buried oxide layer 14 to the oxide layer 18. A bond pad or trace 56 is located above the passive layer 22 and in electrical communication with the connector 48.
The traces of a sensor device, such as the trace 56 of the pressure sensor 10 of FIG. 1, are often formed from a layer of metal deposited on a top surface of the sensor device. The placement of metal on the top surface of the sensor device, however, can lead to aging and drifting of the sensor properties. These property changes are generally due to the lack of mechanical stability of metallic films. Moreover, the metal layer deposited on the top surface of the sensor is typically patterned to provide the individual electrical connections to the various sensor components, thus adding an extra processing step.
What is needed is a capacitive sensor which does not use an applied metal layer to provide electrical connection to electrodes within the device. A sensor that does not use an applied metal layer which can be manufactured with known fabrication processes would be further beneficial.